The present invention relates to a multi-beam exposure apparatus which emits two or more light beams for exposure and which is applicable to a high-speed digital copying machine, a high-speed printer apparatus or the like. The present invention also relates to an image forming apparatus that employs that exposure apparatus.
An image forming apparatus that utilizes an electrostatic photography process includes an image reading section and an image forming section. The image reading section photoelectrically converts optical image information to an electric image signal and outputs this electric image signal. The optical image information represents a light intensity pattern of an object to be read (e.g., a sheet-like document or a book) and is specifically light reflected from the object. The image forming section forms an image corresponding to the image signal obtained by the image reading section. The image forming section forms an image on the basis of not only an image signal supplied from the image reading section but also an image signal externally supplied.
In this type of image forming apparatus, the image forming speed is increased by radiating a plurality of laser beams to a photosensitive body at one time. The laser beams are adjusted such that they have a diameter corresponding to the resolution required of an output image.
There are two method in which a plurality of laser beams are radiated onto the laser beams at one time. One of the methods is to employ a number of exposure apparatuses each radiating a single laser beam. The other method employs a multi-beam exposure apparatus made up of one or two exposure apparatuses. The multi-beam exposure apparatus includes two or more semiconductor laser elements, optical members for adjusting the diameter of the laser beams from the semiconductor laser elements and the positions to which the laser beams are radiated at one time. The former method, which employs a number of exposure apparatuses, is disadvantageous in that the copying apparatus (or printer apparatus) requires a wide installation space for the exposure apparatuses. In addition, the former method inevitably increases the manufacturing cost. This being so, multi-beam exposure apparatuses are in wide use these days.
As described above, the multi-beam exposure apparatus includes two or more semiconductor laser elements each radiating a laser beam, and optical members for adjusting the diameter of the laser beams emitted from the semiconductor laser elements in accordance with the required resolution and for adjusting controlling the distances between the adjacent laser beams in the axial direction of the photosensitive drum and in the direction perpendicular thereto. In addition to these structural elements, the multi-beam exposure apparatus includes a deflecting device for deflecting the laser beams in the axial direction at one time (i.e., scanning the laser beams across the photosensitive body), and a focusing lens for focusing the laser beams deflected by the deflecting device in the axial direction of the photosensitive drum in such a manner that the deflection angle at which the laser beams are deflected by the deflecting device changes in proportion to the distance for which the laser beams travel before reaching the photosensitive body. A method for adjusting the distances between the adjacent laser beams to be predetermined values is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-76704 (i.e., Japanese Patent Application No. 8-233198), for example. According to the publication, a galvano-mirror, which has a mirror surface rotatable in an arbitrary direction, is employed for adjusting the distance between the two adjacent beams. The galvano-mirror is, for example, a mirror device whose mirror surface is driven finely by an electromagnetic coil or the like.
According to Jpn. Pat. Appln. KOKAI Publication No. 10-76704, a dampening agent, such as silicone gel, is used for preventing the mirror from vibrating due to external vibration. A mirror driving force is produced by the interaction between a magnetic force which is provided by a magnetic circuit and a driving force which is applied from an electromagnetic coil to a mirror support member to tilt the mirror at a predetermined angle. This driving force is adversely affected if the viscosity of the dampening agent is lowered by a temperature increase caused by the heat generated by the coil or if the magnetic force provided by the magnetic circuit varies due to the temperature increase. As a result, the relationships between the current flowing through the coil and the angle of rotation of the mirror change with time. Due to this drift phenomenon, the mirror cannot be driven to a desired angle at all times.
Due to the view of the problems discussed above, successive image forming operation may not be performed reliably. To be more specific, the image formed immediately after the power switch is turned on and the image formed immediately before the image forming operation comes to an end may differ in size. In addition, an image formed under special conditions may be distorted. Although image formation for one sheet is completed within several seconds, the condition under which an image is formed may vary in this short period. In other words, an image is not formed under the same condition between the front and rear end portions, and the lines constituting an image may vary in the sheet conveyance direction and in the direction perpendicular thereto.